Merge branch 'sentry' into 'master'

Add Sentry remote error collection

Closes https://gitlab.com/gitlab-org/gitlab-workhorse/issues/57

Vendors:
- https://github.com/getsentry/raven-go (unnamed permissive license)
- https://github.com/certifi/gocertifi (Mozilla Public License 2.0)

Discussed licenses with @JobV; they seem OK.

See merge request !63